Separation of the components of the delayed rectifier potassium current using selective blockers of IKr and IKs in guinea‐pig isolated ventricular myocytes

Abstract

Delayed rectifier potassium current (IK) was investigated in guinea‐pig isolated ventricular myocytes under voltage‐clamp conditions (‘switched’ single electrode clamp), using selective blockers and/or different activation protocols to separate its rapid (IKr) and slow (IKs) components. The class III antiarrhythmic compound E4031 (5 microM) was used to block IKr and the anaesthetic drugs propofol (100 microM) or thiopentone (100 microM) to block IKs. In all experiments L‐type calcium currents were blocked with nifedipine (2 microM). Complementary effects of E4031 and the anaesthetic drugs on the components of IK were observed. The E4031‐sensitive current (IKr) resembled the current remaining in the presence of the anaesthetics and, likewise, the anaesthetic‐sensitive current (IKs) resembled the current remaining in the presence of E4031. Under the conditions of these experiments, the relative contribution of the two components to total IK tail current was found to be approximately equal after a 400 ms depolarization to +40 mV. For example, IKr was 58 +/− 10% of total IK tail current when measured as the E4031‐sensitive current, 41 +/− 6% as the propofol‐insensitive current and 43 +/− 7% as the thiopentone‐insensitive current. In the presence of both E4031 and propofol or thiopentone the IK tail current deactivating at ‐40 mV was completely eliminated, leaving a residual current during the pulse which reversed at ‐46 +/− 1 mV. To avoid complication of the ‘envelope of tails’ test with this residual current, the tail:pulse ratio was calculated for the anaesthetic‐sensitive component and this was constant, consistent with block of a single component of IK. Forskolin (1 microM) enhanced the current most consistent with IKs. Propofol (300 microM) caused a 64 +/− 3% increase in action potential duration in the presence of both E4031 (5 microM) and nifedipine (2 microM), consistent with an important role for IKs in the repolarization of the action potential in the guinea‐pig heart. The observations therefore provide further support for separate components of IK with different characteristics in the guinea‐pig heart; it appears that E4031 and propofol or thiopentone are useful complementary tools for their separation.

Sevoflurane Inhibition of the Slowly Activating Delayed Rectifier K+ Current in Guinea Pig Ventricular Cells

“…The most common mechanism of QT interval prolongation by pharmaceuticals appears to be inhibition of I Kr (2). On the other hand, I Ks is blocked by chromanol derivatives (15,16) and some general anesthetics including propofol and barbiturates (17). Thus, inhalational anesthetics, when administered in the presence of other agents that block I Ks or I Kr , may trigger excessive QT prolongation, depending on the specificity of the anesthetics on I Ks and I Kr .…”

Density and Kinetics of I Kr and I Ks in Guinea Pig and Rabbit Ventricular Myocytes Explain Different Efficacy of I Ks Blockade at High Heart Rate in Guinea Pig and Rabbit

“…1,2 They differ in kinetic properties, rectification characteristics, and sensitivity to drugs. 1,2 Most of the class III antiarrhythmic drugs, such as dofetilide and E-4031, prolong APD in a reverse frequency-dependent manner by blockade of I Kr .…”

“…1,2 They differ in kinetic properties, rectification characteristics, and sensitivity to drugs. 1,2 Most of the class III antiarrhythmic drugs, such as dofetilide and E-4031, prolong APD in a reverse frequency-dependent manner by blockade of I Kr . 2,3 Therefore, these I Kr blockers may act in a proarrhythmic manner during bradycardia, with minimal therapeutic potency against tachyarrhythmias.…”

“…1,2 Most of the class III antiarrhythmic drugs, such as dofetilide and E-4031, prolong APD in a reverse frequency-dependent manner by blockade of I Kr . 2,3 Therefore, these I Kr blockers may act in a proarrhythmic manner during bradycardia, with minimal therapeutic potency against tachyarrhythmias. 4,5 Chromanol 293B has recently been reported to selectively block I Ks .…”

The Dog's Role in the Preclinical Assessment of QT Interval Prolongation

“…This species has been described as having fairly good correlation to human AP prolongation using known agents that lengthen QT interval (22). On a molecular basis, the guinea pig does not appear to express Ito current (18), and the predominant portion of the repolarizing current is due to the slow activating delayed recitifer I Ks (30). Although this channel plays a role in the dog, human, and rabbit, its predominance in the guinea pig distances it from the human with regard to repolarization mechanism of action.…”